Add api-ece variable (#3095)

Author: lcawl

shared/attributes.asciidoc

@@ -203,6 +203,7 @@ Elastic Cloud
 :ess-setting-change: {ess-icon} indicates a change to a supported {cloud}/ec-add-user-settings.html[user setting] for {ess}.
 :ess-skip-section: If you use {ess}, skip this section. {ess} handles these changes for you.
 :api-cloud:  https://www.elastic.co/docs/api/doc/cloud
+:api-ece:    https://www.elastic.co/docs/api/doc/cloud-enterprise
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 Elasticsearch
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